1. Field of the Invention
The present invention relates to voltage clamping circuits, and in particular, to voltage clamping circuits which are self-triggered in accordance with the amplitude of its power supply voltage.
2. Description of the Related Art
As the scale of integration has increased in integrated circuits (ICs) containing N-channel metal-oxide-semiconductor field effect transistors (N-MOSFETs), such circuits have become increasingly prone to latching up during initial circuit powerup sequences. As is well known in the art, parasitic effects of the various semiconductor fabrication processes (e.g., parasitic bipolar junction transistors) can tend to cause the MOSFET circuitry to latch up upon the initial application of its power supply voltage.
One common technique to prevent circuit latchup from occurring has been to apply a biasing voltage to the underlying substrate of the integrated circuit which is negative with respect to the circuit reference, or ground, node. This negative substrate bias, commonly referred to as a "back bias voltage" (VBB), is frequently generated by a charge pump circuit driven by an oscillator. Referring to FIG. 1, an oscillator drives a charge pump with an AC input signal. The charge pump converts the low amplitude AC input signal to a pumped-up DC voltage which is applied to a VBB pad which is connected to the underlying substrate of the IC. (Such back biasing of an IC substrate is well known in the art. See, for example, L. A. Glasser and D. W. Dobberpuhl, "The Design and Analysis of VLSI Circuits," Addison-Wesley, 1985, pp. 301-08.)
In theory, the use of an oscillator and charge pump to provide a substrate back bias can prevent circuit latchup during the initial stages of powerup. However, in practice, problems can develop. Since the oscillator and charge pump are also powered by the IC power supply (VCC), they too are subject to the problems caused by an initially low power supply voltage during the initial stages of powerup. In other words, during the initial stages of powerup, the power supply voltage VCC is at a low, albeit increasing, voltage and the oscillator may not oscillate at a high enough frequency to provide sufficient drive to the charge pump. This can prevent a sufficiently high back bias VBB from being generated in time while, in the meantime, transistor leakage or substrate coupling can cause the IC substrate to become forward biased, thereby resulting in circuit latchup.
Accordingly, it would be desirable to have a circuit which prevents forward biasing of the IC substrate until such time as the back bias oscillator and charge pump are sufficiently operational to supply the necessary substrate back bias voltage VBB.